Coastal areas in Spain are suffering the effects of a Nov. 13 spill that sent millions of gallons of oil into the waters off the nation’s northwestern coast. Although high winds and strong currents have kept most of the spilled fuel oil farther out in Atlantic waters, waves of oil crashed over a 20-foot sea wall along the Galician coast on Sunday, fouling towns and beaches with a layer of oil up to 10 inches thick. Oil slicks also threatened to contaminate fishing waters and imperil the fishing and shellfish industries — the economic base of the region — along 310 miles of coast. The spill occurred when the Prestige, a tanker carrying 20 million gallons of fuel oil, ruptured in a storm and sank. Environmentalists say the tanker has leaked 5.3 million gallons of oil; so far, about 1.5 million gallons have been collected from the sea and another half-million or so from land, and damages are estimated at $42 million. Some 200,000 Galicians marched in protest on Sunday, claiming the government responded too slowly to the spill.